This invention relates generally to fiber optic face plates and more particularly concerns methods in which a photoform glass can be etched and then the etched portions filled with either a melted low index glass, plastic, or a dark matrix material for fabrication of fiber optic faceplate equivalents.
Fiber optic faceplates (FOFPs) are useful in the construction of liquid crystal displays as disclosed in U.S. Pat. No. 5,442,467, filed on Mar. 21, 1994, by Silverstein et al., the subject matter of which is incorporated herein by reference. U.S. Pat. No. 5,442,467 discloses a direct-view rear-illuminated LCD device, comprising: a backlight source; a rear diffuser layer; a rear polarizer; a LC cell including a rear glass layer with addressing elements and indium tin oxide (ITO) transparent pixel electrodes, a LC layer having a top and bottom surface, and a front FOFP as a front containing element of the LC cell and being located directly in contact with the top surface of the liquid crystal layer; a mosaic array of color absorption filters either deposited on the front face of the FOFP or located on a separate but adjacent substrate; and a front polarizer or analyzer. Alternatively, the front polarizer or analyzer may be constructed from thin-film materials and located between the top or light exit surface of the LC layer and the bottom or light input surface of the FOFP.
An FOFP comprises an array of individual optical fibers which are fused together with an interstitial cladding material and then cut and polished to a desired thickness to form a plate. The creation of FOFPs with varying optical characteristics is well known in the art. The optical fibers are designed to transmit through total internal reflection light incident at controlled input or acceptance angles while rejecting or absorbing light incident at larger angles. Light entering the fibers at an entrance plane of the FOFP is collected over a wide acceptance angle .theta..sub.Max IN by use of a high numerical aperture (NA) FOFP and/or coupling to a boundary of low refractive index (e.g., air). Light exiting the optical fibers of an exit plane of the FOFP is made to diverge or exit over a relatively wide angle .theta..sub.Max OUT also by use of a high NA and/or the ultimate coupling to a low refractive index boundary. FOFPs with low NAs and/or coupling to relatively high refractive index materials (e.g., plastic, polyimide, or optical glass) restrict the light output exit angle, .theta..sub.Max OUT, of the exit plane of the FOFP and the light input acceptance angle, .theta..sub.MAX IN, of the entrance plane of the FOFP, respectively.
These relations are illustrated in FIG. 1 for a typical optical fiber 10. Light beam 16 enters the optical fiber 10 within the acceptance cone 20 defined by an angle .theta..sub.max, which is measured from normal line N, and is totally internally reflected within a core 12 of the optical fiber 10 to propagate down the length of the optical fiber 10, essentially without loss. The normal N is perpendicular to an entrance plane 30 and an exit plane 32 of the optical fiber 10. If the relative index of material surrounding the optical fiber 10 at the entrance plane 30 and exit plane 32 surfaces (N.sub.o) is the same, then the light beam 16 will exit the optical fiber 10 at the same angle, in this example .theta..sub.max, which it entered. Light beam 18, which enters the optical fiber 10 outside of the acceptance cone 20 defined by .theta..sub.max is not fully guided through the length of the optical fiber 10 and "leaks" out of the optical fiber 10 into adjacent cladding material 14. Light beam 16 is a guided light beam while light beam 18 is an unguided light beam. An unguided or partially guided light beam may pass through the cladding material 14 and enter other fibers in a fiber-optic bundle or fused faceplate. However, unguided or partially guided light beams typically leak out of these fibers as well and continue to traverse the bundle or faceplate.
FIGS. 2 and 3 show the effects of varying the numerical aperture of the optical fiber 10. FIG. 2 shows the optical fiber 10 having a small numerical aperture and thus a smaller light acceptance cone 20. FIG. 3 shows the optical fiber 10 having a large numerical aperture and thus a larger light acceptance cone 20. Thus, the higher the numerical aperture of the fiber 10, the larger .theta..sub.max at the entrance plane 30 and the exit plane 32.
In general, light which enters the optical fiber 10 is rotated about a central axis of the optical fiber 10 as it propagates along the length of the optical fiber 10 as shown in FIG. 4. In this example the central axis of the optical fiber 10 happens to be coincident with the normal N used to measure the angle .theta..sub.max. Thus, light which enters at a given angle from the normal N to the fiber input surface exits the optical fiber 10 at the same exit angle, but at a rotated azimuthal position. This rotation is dependent on the number of reflections within the optical fiber 10 and also by the internal surfaces of the fibers. Skew rays typically undergo more rotation than meridional rays. For the application of FOFPs to LCDs, most of the illumination entering the fiber will be skew rays.
In FIG. 4, a light ray 24 and a light ray 26 can be seen entering the optical fiber 10 at the entrance plane 30 at an angle .theta..sub.max measured with respect to a normal N. Light ray 24 is parallel to light ray 26 and they enter the optical fiber at different points on the entrance plane 30. As each light ray 24, 26 exits at the output plane 32 of the optical fiber 10, it can be seen that each light ray 24, 26 exits at an angle .theta..sub.max but having undergone an azimuthal rotation angle .phi. around the central axis of the optical fiber 10.
As explained above, in fused fiber optic bundles and faceplates, both guided and unguided rays undergo azimuthal rotation. As shown in FIG. 4, the consequence of this rotation is that the optical fiber 10 averages about the azimuthal position all of the incoming light entering at a given declination angle such that the output consists of a hollow exit cone 22 with a solid angle of twice the entrance angle. In FIG. 4, because both illustrated incoming light rays 24, 26 enter the optical fiber 10 at an angle .theta..sub.max, the solid angle of the hollow exit cone 22 is .theta..sub.max. As the light emerging as a hollow exit cone 22 consists of an average about the azimuthal position, the transmitted light intensity is equal at all azimuthal angles. It is this property of azimuthal averaging that enables FOFPs to produce symmetrical viewing characteristics over wide angles when coupled to a LCD with inherent anisotropies in luminance and contrast.
FIG. 5 illustrates an FOFP 28 made of an array of individual optical fibers which are fused together with an interstitial cladding material and then cut and-polished to a desired thickness to form a plate. The core 12 and cladding material 14 can be seen on the surface of the FOFP 28.
Therefore, any plate which has columnar features approximately in the direction of light propagation which are capable of total internal reflection, a controllable numeric aperture (NA) at input and output surfaces, rotational azimuthal averaging and translation of the object plane from a back surface of the plate to a front surface of the plate is the optical equivalent of a FOFP.
A further improvement to FOFPs is discussed in U.S. patent application Ser. No. 08/473,887, filed Jun. 7, 1995 by Silverstein et. al. and titled "Enhanced Off-Axis Viewing Performance of a Liquid Crystal Display Employing a Fiberoptic Faceplate Having Masked Interstitial Apertures" and herein incorporated by reference.
Diffraction is the deviation from rectilinear propagation that occurs when light waves advance beyond any obstruction or boundary. The obstruction may be opaque, as in the case of a knife-edge or pinhole, or may be a boundary defined by two transparent materials with different refractive indices. Since light reflects, refracts or diffracts from a straight path when encountering a boundary or obstruction, the intensity distribution of a point of light which undergoes diffraction, when projected on a surface some distance from the boundary, will be characterized by a spread function or diffraction pattern. For light transmitted through an aperture, the degree of diffraction or angular deviation in the path of light is determined by the size and shape of the aperture and the wavelength(s) of light from the source. The diffraction pattern at some remote position from the aperture is additionally a function of the distance from the aperture to the plane of observation. The remote or far-field diffraction pattern is typically referred to as a Fraunhoffer diffraction pattern. In optical systems where the circular apertures of lenses, stops and pupils are typically constraints, the Fraunhoffer diffraction pattern is often referred to as the Airy disk. The Airy disk arising from light passing through a circular aperture is well described by a first-order Bessel function with a central bright region surrounded by a series of faint rings of rapidly diminishing intensity. Approximately 84% of the light intensity from a diffracted point source is contained within the first dark ring of the Airy disk. As such, the Airy disk characterizes the blur circle produced by diffraction-limited optical systems.
In assessing the impact of diffraction on FOFPs, U.S. patent application Ser. No. 08/4,673,887, by Silverstein et. al. filed on Jun. 7, 1995 and assigned to Xerox Corporation and which is herein incorporated by reference, focuses on the angular dispersion of light incident on the FOFP. As shown in FIG. 6, the FOFP consists of a fused plate of optical fibers consisting of a core 12 and interstitial cladding 14, which constitute two distinct populations of very small apertures. Both the entrance plane 30 and the exit plane 32 of the cores 12 can be considered as small circular apertures. The cladding 14 on the entrance and exit plane 30, 32 surfaces are somewhat irregular in shape and size. However, for purposes of discussion, the cladding 14 will be described as circular apertures with a diameter estimated from the mean diameter of all claddings 14. Guided ray 16 entering the FOFP is diffracted into an angular distribution of light paths. The degree of diffraction and hence the width of the angular distribution of light paths is inversely proportional to the diameter of the aperture. Thus, the smaller the aperture the larger the angles into which light propagation through the FOFP is diffracted. The cladding 14, being significantly smaller than the core 12, diffracts the incoming light into the largest angles. FIG. 6 also shows the relative Fraunhoffer diffraction patterns or Airy disks 38 which would result from the projection of the core 12 and cladding 14 diffraction angle distributions on the retina of an observer 34 located some fixed distance from the FOFP. The angular spread resulting from diffraction can be estimated from the following equation: EQU q.sub.diffr =1.22[(.lambda.)(180)]/[(D)(.pi.)]
where:
q.sub.diffr =half-angle corresponding to first dark ring of the Airy pattern (degrees)
D=diameter of circular aperture
.lambda.=wavelength of light
By reference to the above equation and assuming nominal core 12 and cladding 14 diameters of 7 microns and 0.5 microns, respectively, it can be estimated that for incoming light of 550 nm, the diffraction angle corresponding to the first dark ring of the Airy disk 38 is approximately 5.49.degree. for the core 12 apertures and 76.9.degree. for the cladding 14 apertures. For on-axis illumination and on-axis viewing of a LCD with coupled FOFP, the effects of diffraction in the FOFP will be primarily manifested as a small reduction in display luminance. This is in large part a result of the small light acceptance cone of the eye and of most photometric measurement instruments.
FOFP diffraction is responsible for anomalous reductions in on-axis contrast for coupled LCDs. Establishing this causal relationship would enable the development of effective means to reduce these observed reductions in on-axis LCD contrast. To describe this problem, consider the angle-dependent contrast performance of typical twisted-nematic (TN) or super-twisted nematic (STN) LCDs that has been previously described. The contrast ratio of such displays is typically very high when observed on-axis but exhibits a progressive degradation at off-axis viewing and light propagation angles. This observed contrast degradation, while progressive, is not isotropic for the reasons previously described. At some extreme angles, the contrast of the display may actually reverse resulting in a negative image. These off-axis contrast degradations do not affect the high on-axis contrast performance of the display due to the small light acceptance cone of the eye or of most photometric measurement instruments. However, when a FOFP is coupled to such an LCD, the on-axis contrast performance of the FOFP-coupled display is substantially reduced below the levels achieved without the FOFP. Improvement in the on-axis contrast performance of FOFP-coupled LCDs provide an important enhancement.
For light propagating at off-axis angles to contaminate the on-axis contrast performance of an LCD with FOFP, the angular direction must be changed such that some of this light gets coupled into the small light acceptance cone of the eye or measurement instrument. FIG. 7 shows the guided ray 16 emerging from the source (i.e., the backlight) at an angle which is off-axis from the normal N to the FOFP input surface. At the exit plane 32, the light is diffracted by the core 12 apertures and the cladding 14 apertures with an angular distribution about the direction of light propagation. For the larger core 12 apertures, the relatively small diffraction angles do not diffract much light into the light acceptance cone of the observer 34 or instrument. However, for the much smaller cladding 14 apertures, the angular distribution of diffracted off-axis light is quite large and a significant amount of the off-axis light is diffracted into the small light acceptance cone of the observer 34 or measurement instrument. In this manner, the off-axis light from the LCD (and corresponding contrast degradations) are diffracted by the FOFP cladding 14 apertures into the small light acceptance cone of the observer 34 or instrument resulting in significant degradation of on-axis contrast performance of the FOFP-coupled LCD.
The on-axis contrast performance of the FOFP-coupled LCD can be dramatically improved by masking the cladding 14 apertures of the FOFP as shown in FIG. 8. This figure illustrates a FOFP with masked cladding 36 and how such masking prevents the cladding 14 apertures from diffracting off-axis light into the observer's viewing cone. Evaluations of LCDs coupled to FOFPs with masked cladding 36 have confirmed the effectiveness of this enhancement, resulting in dramatic improvements in the on-axis contrast performance of FOFP-coupled LCDs. These analyses and the resulting FOFPs with masked cladding 36 provide significant enhancements to the invention disclosed in U.S. Pat. No. 5,442,467 the subject matter of which is incorporated herein by reference.
These essential optical properties can be imparted to a range of materials, thus producing the FOFP optical equivalents. This application discusses a method employing irradiation sensitive glasses and creating adjacent areas with differing refractive indices which result in a substrate containing a plurality of cylindrical features whose boundaries are defined by a discontinuity of refractive indices wherein the index of refraction within the cylindrical features is greater than the index of refraction at the boundaries and external to the cylindrical features. In an alternative embodiment a substrate containing a plurality of cylindrical features whose boundaries are defined by a light blocking material is created.
Accordingly, it is the primary aim of the invention to produce substrates containing a plurality of cylindrical features whose boundaries are defined by a discontinuity of refractive indices wherein the index of refraction within the cylindrical features is greater than the index of refraction at the boundaries.
Further advantages of the invention will become apparent as the following description proceeds.